1. Field of the Invention
The present invention relates to friction material used for a synchronizer ring in a speed change gear, for example in a syncromesh transmission.
2. Discussion of the Prior Art
A synchronizer ring is a ring-shaped member assembled, for example, in a speed change gear of an automobile for synchronously rotating two gears which are changed and meshed upon gear changing operation of the speed change gear such that the two gears engage smoothly with each other.
The synchronizer ring has a friction material layer secured to a ring-shaped structural body. The friction material layer is disposed to the inner circumference of the structural body, to the outer circumference of the structural body or on both sides of the inner circumference and outer circumference of the structural body. In each case, the friction material layer is frictionally engaged to a tapered portion (cone portion) of the speed change gear.
As a friction material for the friction material layer, copper alloys, for example, MBA-2 and MBA-5 are often used. Further, a paper material is also used although not frequently. In addition, molybdenum is sometimes secured by flame spraying to the structural body.
The following characteristics are required for the synchronizer ring:
(1) Having a large dynamic friction coefficient to a mating member for synchronizing two gears by frictionally engaging a tapered portion as the mating member. PA1 (2) Having a low friction coefficient at a relative speed nearly equal zero (hereinafter referred to as "static friction coefficient") in order to decrease impact shocks upon engaging a mating gear. PA1 (3) Being resistant to scorch as a result of braking frictional heat when braking is applied to a mating member, and particularly being resistant to scorch from braking frictional heat generated upon misoperation of a shift lever.
The misoperation of the shift lever referred to above occurs when the shift lever is displaced without properly pedaling the clutch pedal. Upon this misoperation, since the engine torque is transmitted to one gear and the other gear is rotated at a torque greater than the torque of idling when the clutch is completely disengaged, a large thermal load (reaching ten and several times as much as the usual load) is applied to the synchronizer ring. As a result, the temperature of the friction material is elevated by braking frictional heat, possibly causing carbonization or abnormal wear to deteriorate the synchronizing function.
Copper alloys are often used as the friction material since precedence tends to be given to the characteristic (3) among the three characteristics described above. However, since copper alloys have a relatively low dynamic friction coefficient, the characteristics (1) and (2) which copper alloys exhibit are poor.
In view of this, the friction surface of the copper alloy has been made as dual or triple composite layers, thereby increasing the capacity for absorbing the braking frictional heat and increasing the dynamic friction coefficient, to improve characteristic (1). However, if the friction surface is made in composite layers, it results in the problem of the mechanism of the speed change gear being complicated, enlarged in scale and expensive.
On the other hand, although a paper material has characteristics (1) and (2), it is susceptible to scorch by braking frictional heat upon misoperation of the shift lever. Further, although molybdenum exhibits good dissipation of braking frictional heat upon misoperation of the shift lever, it performs relatively poorly respecting characteristics (1) and (2).
In addition, to provide the synchronizer ring with a satisfactory friction characteristic, it is necessary to form a number of fine holes to the friction material. If a number of fine holes are formed in the friction material, when the synchronizer ring is brought into contact with a mating member, the formation of oil membranes at the friction interface is reduced, thereby increasing the dynamic friction coefficient. Thus, the friction material used in a synchronizer ring should have good heat resistance, wear resistance, scorching resistance and porosity, together.